Benthic ecosystem functioning in hydrocarbon and heavy-metal contaminated sediments of an Adriatic lagoon

Sediment samples were collected in 4 sites along a salinity gradient (between 0.2 and 27.3) of the Aussa River (Marano Lagoon, northern Adriatic Sea, Italy) and a decreasing gradient of industrial contamination downstream (from Stn A1 to Stn A4) to assess how a riverine-lagoonal system responds to chemical contamination by applying an integrative measure of its benthic ecological properties. Benthic biodiversity (primary producers, meiofauna, macrofauna) along with other structural and functional parameters were related to contamination. Meiofaunal abundance varied between 176 ± 65 and ~4000 ind. 10 cm−2 downstream. The macrofaunal composition shifted from one composed exclusively of chironomid larvae to a typically lagoonal one. At Stn A1, the benthic microalgal abundance exceeded 500 000 cells cm−3 and seemed unaffected by contamination. In the freshwater-impacted site, primary production was one order of magnitude higher than in the more saline one (7.20 and 0.60 μg C cm−3 h−1, respectively). High organic carbon enrichment (65 mg C gdry) at Stn A2 probably caused the highest community respiration, estimated by O2 microprofiles, along with the highest exoenzymatic activities (β-glucosidase, leucine aminopeptidase and lipase). The 4 investigated sites were well separated by principal component analysis along a decreasing gradient of organic load. An extremely active microbial community in the contaminated site ensured high production and degradation rates. Correct assessment of the benthic ecosystem functioning in polluted areas may represent a useful tool for transitional system management in order to plan environmental interventions in a rational way.